1. Field of the Invention
The present invention relates to a solid electrolyte gas concentration detector which is mounted on an exhaust pipe of an internal combustion engine to detect concentration of gas constituents included in the exhaust gas.
2. Description of Related Art
Various technologies for controlling exhaust gas emission from an internal combustion engine have been known and used hitherto. For example, an air-fuel mixture supplied to an engine is controlled to be stoichiometric, and an exhaust gas from the engine is sent to a catalytic converter having three-way catalyst in which emissions such as hydro-carbon (HC), carbon-monoxide (CO) and nitrogen-oxides (NOx) are converted into harmless components. Recently, exhaust emission regulations have become stricter (as in OBD-II regulations), and a self-diagnostic system which gives warning to a driver when emission control devices including the three-way catalytic converter malfunction has to be installed on an vehicle. The driver is also required to repair the malfunctioning devices.
As a technology for detecting deterioration of the three-way catalytic converter, a two-O.sub.2 -sensor system is known in which one O.sub.2 sensor is disposed upstream of the converter and the other O.sub.2 sensor is disposed downstream thereof. However, as the emission regulations become much stricter from LEV (low emission vehicle) to ULEV (ultra low emission vehicle), the two-O.sub.2 -sensor system which detects indirectly a cleanup rate of the converter from signals of two O.sub.2 sensors cannot detect it with a sufficiently high accuracy. Therefore, it is necessary to provide a gas concentration detector which directly detects components (HC, CO and NOx) in the emission with a high accuracy. It is also necessary to provide a gas concentration detector which can specifically detect methane (CH.sub.4), because an engine which burns fuel including methane as a principal component (CNG engine) is being developed recently. Moreover, a gas concentration detector which is able to detect CO gas is required not only for the exhaust emission control system for an internal combustion engine but also for a fuel cell to detect CO coming out therefrom when methanol is decomposed.
Some kinds of gas detectors that directly detect gas concentration have been proposed. Such detectors utilize a oxygen ion conductive solid electrolyte, on both surfaces of which an electrode exposed to a reference gas and an electrode exposed to measuring gas are disposed, respectively, and voltage appearing between the electrodes is used for detecting gas concentration. This kind of gas detector detects the gas concentration from an amount of oxygen generated or consumed in the process of oxidization or reduction of the gas constituents. Since the oxidization or reduction in this kind of the detector is not selective to a specific gas constituent, oxygen concentration in the measuring gas has to be kept constant.
One of the gas detectors of this kind utilizing a solid electrolyte is disclosed in an SAE (Society of Automotive Engineers) paper, No. 960334. This is a thick film ZrO.sub.2 sensor for detecting NOx gas. A oxygen detecting cell and a first pumping cell for exhausting oxygen are disposed in a first chamber into which measuring gas is introduced. A second pumping cell is disposed in a second chamber communicating with the first chamber through a diffusion resistance and driven by a predetermined voltage to exhaust oxygen in the chamber to an outside space. An electrode of the second pumping cell in the chamber is active for reducing NOx. Oxygen concentration change in the first chamber is detected by the oxygen detecting cell disposed therein and fed back to the driving voltage of the first pumping cell. In the second chamber, oxygen is newly generated by decomposition of NOx, and pumping current of the second pumping cell increases or decreases according to the concentration of NOx. The concentration of NOx is detected by measuring the pumping current. When this gas detector is used for detecting HC or CO, it is influenced by oxygen concentration in the chamber and calibration to eliminate oxygen influence is necessary because oxygen is consumed to oxidize HC or CO. In other words, this gas detector cannot avoid the influence of oxygen concentration in the measuring gas.